Injection molding method for plastic materials

ABSTRACT

In an injection molding method for plastic materials, an injection mold is used which includes two mold halves incorporating thinwalled members to define a cavity therebetween. Before injecting plastic material into the cavity, the mold is heated to a temperature above the melting point of the plastic material by circulating a heat carrier flowing through a heating device. During injection of plastic material, the flow of heat carrier is stopped for maintaining the temperature of the mold and for supporting the thinwalled members. After completely filling the cavity, the mold is cooled down to a temperature below the freezing point of the plastic material by suitably circulating the heat carrier which now passes through a cooling device. The circulation of the heat carrier is carried via two separate circulation systems, with one being close to the inner core of the mold and the other one being arranged in proximity of the sprue so that the cooling can be controlled by starting in an area distant to the sprue and progressing toward the sprue in a time-controlled manner.

BACKGROUND OF THE INVENTION

The present invention refers to an injection molding method for plasticmaterial and to an injection mold for carrying out the method.

In general, an injection mold has a cavity in which the fluid plastic isforced and allowed to solidify to reproduce the shape of the mold. Inorder to heat and cool the mold, a heat transfer medium or heat carriermay be used which flows through respective channels. In particular, whenmolding highly sensitive plastic material with critical flow behavior,problems were encountered as far as dimensional stability of the moldingwas concerned. Also the making of thinwalled moldings for which longflow passages are required in the injection mold has proven to bedifficult and uneconomic.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide an improvedinjection molding method for plastic material and an improved injectionmold for carrying out the method obviating the afore-stated drawbacks.

This object and others which will become apparent hereinafter, areattained in accordance with the present invention by raising thetemperature of the injection mold above the melting point of the plasticmaterial through circulation of a heat carrier before injecting plasticmaterial into the injection mold, by shutting off the flow of the heatcarrier through the injection mold upon injection of plastic materialinto the injection mold, and by cooling the injection mold to atemperature below the freezing point of the plastic material throughsuitable circulation of the heat carrier after the cavity is filled withplastic material.

An essential feature of the present invention resides in the fact thatat least those parts which define the cavity and come into contact withthe material to be molded are heated to a temperature above the meltingpoint so that the material in the cavity is continuously kept in aflowing state. This ensures that the cavity is completely filled withmaterial also in the most distant areas even when long flow passagesexist.

Suitably, the flow of heat carrier through the injection mold isinterrupted when injecting material into the cavity so that thetemperature of the walls of those parts defining the cavity ismaintained at the desired level, and at the same time, the passagewaysthrough the injection mold which are filled with the heat carrier areused for support during introduction of the material.

According to a further feature of the present invention the sprue areais heated and cooled separately from the core area of the injectionmold. Preferably, two separate heat carrier circulations are provided.Heating and cooling is suitably attained by a same liquid fluid such asoil, with the circulations being selectively connected to an externalheating device and an external cooling device.

An injection mold according to the invention includes inserts which areincorporated into the injection mold and define the cavity. As inserts,two separate thinwalled members are provided which are suitably spacedfrom each other. These thinwalled members can be accurately made to sizeseparate from the other mold parts and can easily be exchanged when wornout without abolishing the entire injection mold as the inner mold coreand the outer parts of the mold can still be used.

In order to reduce a heat loss within the injection mold and to ensure aheating especially of the thinwalled members which define the cavity,the latter are preferably heat-insulated from the outer parts of theinjection mold. The heat insulation may be attained by e.g. air gapswhich surround the thinwalled members.

According to yet another feature of the present invention, the coolingof the molding after injection into the cavity is controlled in such amanner that initially the area most distant to the sprue area is cooled,and then the cooling gradually progresses from that distant area to thesprue area. By using two separate circulations, the cooling process canbe carried out in a time-controlled manner. In order to ensure that theheat carrier completely flows about the sprue area, the outlet and theinlet of the external heat carrier circulation are preferably arrangeddiametrically opposite to each other in the injection mold.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will now be described in more detail with reference to theaccompanying drawing in which:

The sole FIGURE shows a schematic sectional view of an injection moldingapparatus with an injection mold in accordance with the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawing, there is shown a schematic sectional viewof an injection molding apparatus which includes an injection moldingmachine 1 generally designated by reference numeral 1. The injectionmolding machine 1 has a discharge end 2 by which plastic materialtreated in the machine 1 is fed into an injection mold generallydesignated by reference numeral 3.

The injection mold 3 is parted along a plane A--A and includes a moldhalf 5 which has two outer parts 21 and a mold half 6 which has twoouter parts 28. During injection molding, the mold halves 5, 6 arepressed against each other for closing the mold 3. Provided in the mold3 is a cavity 7 which for ease of illustration is shown as filled withplastic material 8. The cavity 7 is defined by an inner thinwalledmember 9 and an outer thinwalled member 10 which are spaced from eachother to provide a space therebetween which represents the cavity 7.

As shown in the FIGURE, the inner member 9 is an insert which extends inthe mold half 5 and partly in the mold half 6 and forms a part of anouter jacket 13 which surrounds an internal core 12 of the mold 3. Withits end remote to the cavity 7, the inner thinwalled member 9 is fixedlyattached to the core 12 in the area of the mold half 5. The inner member9 is spaced from the core 12 so that a passageway or channel 14 isdefined therebetween which is part of an internal heat carriercirculation generally designated by reference numeral 17. The channel 14thus extends between the inside wall surface of the inner member 9facing away from the cavity 7 and the core 12 so as to encase the core12 and to line the wall surface of the inner member 9 distant to thecavity 7. Traversing the inner core 12 at about a central area thereofis a return channel 15 which is connected at one end to the channel 14and at the other end to an outlet 16. The inlet of the inner heatcarrier circulation 17 is designated by reference numeral 18 and isconnected to the channel 14. As heat carrier medium, a liquid fluid suchas oil may be used.

The outer member 10 is also an insert which is arranged within the moldhalf 6 and is connected at the ingate area of the mold 3 in one piecewith a sprue bushing 11 through which plastic material is injected intothe mold 3 from the discharge end 2 of the machine 1. As shown at theright hand side of the FIGURE, the external wall surface of the outermember 10, i.e. the wall surface distant to the cavity 7, extends at adistance to the surrounding outer mold parts 28 so that a channelnetwork 23 is defined comprising a channel 24 which surrounds and linesthe exterior wall surface of the outer member 10 and communicates with achannel 25 surrounding the sprue bushing 11. The channel network 23 ispart of an external heat carrier circulation which is generallydesignated by reference numeral 22 and further includes an inlet 26which is in communication with the channel 24, and an outlet 27connected to the channel 25. Preferably, the same heat carrier as incirculation 17 is used for flowing in the circulation 22.

As shown in the FIGURE, the inlet 26 and the outlet 27 are locateddiametrically opposite to each other to ensure that the heat carrierevenly flows through the channels 24, 25 despite their differentdiametrical dimensions.

The internal heat carrier circulation 17 and the external heat carriercirculation 22 are each in communication with a cooling device 34 and aheating device 35 which for ease of illustration are shown only as boxesand controlled by suitable control valves 36, 37.

A suitable heat insulation between the inner member 9 and thesurrounding mold parts 21 is attained by air gaps 20. Likewise, theouter member 10 is heat-insulated from the surrounding mold parts 28 byair gaps 29. Additional air gaps 38 are provided to avoid heat transferbetween the mold halves 5 and 6. Certainly, the use of insulants forproviding a heat insulation in these areas other than those describedshould be considered within the scope of the present invention.

In order to allow the molding to be extracted, the injection mold 3 isprovided with an ejector generally designated by reference numeral 30.The ejector 30 includes an ejector plate 31 which supports a pluralityof ejector pins 32 for ejecting the finished molding W from the mold 3.The ejection of the finished molding W is obtained by separating themold halves 5, 6 from each other in the area of their parting line A--Aas indicated by the respective double arrows. When moving the moldhalves 5, 6 apart, the molding W adheres to the inner member 9 in themold half 5, and the ejector pins 32 which traverse respective passagesin the mold 3 then push the molding W off the inner member 9. Afterejection of the molding W from the mold 3, the mold halves 5, 6 aremoved toward each other to close the mold 3 while the ejector 30 isreturned to its initial position.

After having described the elements of an injection molding apparatus inaccordance with the invention, its mode of operation will now be setforth in more detail.

Plastic material, for example highly sensitive plastic material likeinjectable fluorinated plastics such as PFA, PVDF, are prepared in theinjection molding machine 1. These plastic materials have a meltingtemperature which ranges in the area of 350° C. up to 400° C. Certainly,other plastic materials may be treated in the injection molding machine1 as well, especially those with critical flow behavior. Duringpreparation of plastic material in the machine 1, the temperature of theinjection mold 3, especially in the areas of the inner and outer members9, 10 is raised to a level which exceeds the melting point of theplastic material to be molded. The heating is carried out by connectingthe internal circulation 17 and the external circulation 22 to theheating device 35 to heat up the heat carrier which then circulatesthrough the passageways of the circulation systems 17, 22 until theinjection mold 3 reaches the desired temperature. Since the heat carrierespecially flows in the area of the inner and outer members 9, 10 of themold 3, these parts are rapidly heated up to the desired temperature inan energy-saving manner.

After preparing the plastic material in the injection molding machine 1and raising the temperature of the mold 3 to the predetermined level,the plastic material is injected into the cavity 7 of the mold 3 via thesprue bushing 11. At beginning of the injection of plastic material intothe mold 3, the circulations 17, 22 are disconnected from the heatingdevice 35, and the heat carrier is prevented from further flow. Thus,the channels of the internal and external circulations 17, 22 are fullyfilled with heat carrier so that the thinwalled inner and outer members9, 10 are supported and reinforced to counter a possible deformation ofthe members 9, 10 during injection of the material into the cavity 7.

Since the temperature of the mold 3 is raised, preferably above themelting temperature of the plastic material injected into the cavity 7,the injected plastic material completely fills the cavity 7 also in itsoutermost areas. Immediately after filling the mold 3 with plasticmaterial, the injection step is stopped and the circulations 17, 22 areconnected to the cooling device 34 in order to attain a controlledcooling down of the plastic mass within the cavity 7 of the mold 3. Bycontrolling the cooling of the molding W, the danger of shear fractureis essentially prevented and the molding W is characterized by aconsiderable dimensional stability.

The cooling of the molding W is preferably started in the area of thecavity 7 which is at greatest distance from the sprue bushing 11.Therefore, only the internal circulation 17 is initially connected tothe cooling device 34 so that the cooling starts at the most distantarea to the cavity 7 and, in a time-controlled manner, graduallyprogresses to the area of the sprue bushing 11 whereby the externalcirculation 22 is then also connected to the cooling device 34.

By using two separate circulation systems 17, 22 for the heat carrier,the cooling down of the molding W in the cavity 7 can be controlled insuch a manner that a material shrinkage during lowering of thetemperature below the freezing point is compensated. Thus, the molding Wis made at considerable dimensional stability. Even when the cavity 7contains long connecting passages, and even when making thinwalledmoldings W, the controlled cooling process in accordance with thepresent invention leads to homogenous and high-quality moldings W. Theprocess steps can be individually adjusted to the plastic material to betreated without requiring a number of test runs of the entire plant.These test runs allow a determination of optimum operating conditionsalso with regard to the time frame.

Through the provision of air gaps 10, 29 for heat insulating the innerand outer members 9, 10 against the external mold parts 21, 28, thedesired temperature can be reliably controlled and heat losses can bereduced. The mold 3 can be quickly heated up to the desired, relativehigh temperature so that the cycles of the injection molding process canbe effectively tuned to the injection molding machine 1.

While the invention has been illustrated and described as embodied in aninjection molding method for plastic materials and in an injection moldfor carrying out the method, it is not intended to be limited to thedetails shown since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

I claim:
 1. An injection molding method for plastic materials,comprising the steps of:providing an injection mold defining a cavity;raising the temperature of the injection mold above the melting point ofthe plastic material before injecting the plastic material bycirculating a heat carrier through the injection mold; shutting off theflow of the heat carrier through the injection mold upon injection ofplastic material into the injection mold; and cooling the injection moldto a temperature below the freezing point of the plastic material bycirculating the heat carrier after the cavity is filled with injectedplastic material.
 2. A method as defined in claim 1, with a sprueconnected to the cavity, wherein said cooling step includes starting thecooling in an area distant to the sprue and progressively continuing thecooling in a time-controlled manner until reaching the sprue area.
 3. Amethod as defined in claim 1, with the injection mold including a spruearea and a core area, wherein the sprue area is heated and cooledseparately from the core area of the injection mold.